Ideal Tool is "right on" with his answer but I'm going to take it a step further and hang my neck out here - as he stated, Colt never made a '51 Navy in 44 for production. I have a feeling you are asking in regards to "repros" - if I'm wrong in my next comment - somebody please correct me. While technically the Colts were the same frame size - just remember that in "repros" - the actual dimensions may vary depending upon the manufacturer, date of manufacturer, etc. In other words - a frame or cylinder arbor from say a ASM '51 Navy in 36 caliber may not necessarily be interchangeable with the frame and cylinder arbor of a Pietta '51 Navy that is 44 caliber. It sounds like you might possibly have a "project" you are working on and if you have specific questions about manufacturers and whether parts will interchange, I'd suggest you drop Doc Hoy a pm - he's worked on a lot of different makes and models or perhaps someone else as well can answer your question and maybe help prevent some added expense in regards to obtaining wrong parts that won't fit. And like I said, if my remarks are "off base" - just ignore them - most folks ignore me anyway!